<style>
    .left {
        width: 200px;
        float: left;
        background-color: pink
    }
    
    .right {
        width: 200px;
        float: right;
        background-color: pink
    }
    
    .center {
        margin: 0 200px;
        background-color: lightblue
    }
</style>

<div class="left">左边固定宽度</div>
<div class="right">左边固定宽度</div>
<div class="center">中间自适应</div>

</head>

<body>

    </html>